Recently, to make power electronics equipment more compact and higher efficiency, the carrier frequency is increasing to MHz order by applying the gallium nitride (GaN)—Field Effect Transistor, etc. In this case, resonance phenomena occur due to the influence of stray capacitance in the windings of magnetic devices. In this chapter, the high frequency (HF) behaviors of a multi-turn coil and a ring core inductor are simulated by using the electromagnetic field finite element analysis taking account of the stray capacitance. It is shown that the resonance phenomena can be simulated well. Moreover, to clarify the mechanisms of the resonance phenomena in the coil and inductor, equivalent circuit models are proposed correspondingly and verified.